The present invention relates to oxide superconductors containing a rare-earth metal and metals of the first and second groups of the periodic system, and to manufacturing methods therefor.
The quality of superconductors is essentially characterized by: initial (T.sub.initial), mean (T.sub.m) and final (T.sub.final) temperatures in transition to a superconducting state; a temperature transition range (.DELTA.T); a critical current density; and time stabilizes of said characteristics.
The known LnBa.sub.2 CuO.sub.6,5-7 oxide superconductors, in which Ln is Sc, Y or a metal of the lanthanide series, have T.sub.initial from 40 to 95K, T.sub.final from 26 to 75K and a fairly wide transition range (up to 50K). As a rule, such superconductors are characterized by low quality, inclusion of source constituents in addition to a superconducting phase with resultant spread in T.sub.initial, T.sub.final and .DELTA.T, and time instability (impaired superconducting characteristics due to the effect of air in storage). Moreover, this withstand only small critical currents.
They are pressed black materials having a particle size of 1 to 30.mu.. Density of the final product varies.
They are produced by heat treatment of the La, Ba and Cu source oxides. The powdered La.sub.2 O.sub.3 :BaO and CuO oxides are normally compacted after vigorous agitation, placed in an electric furnace and heat-treated at 900.degree. to 1100.degree. C. for a fairly long time period.
The prior-art methods of obtaining superconductors have been generally unsatisfactory due to great power consumption, a long and multistage production process, low efficiency and small output of a superconducting phase.
For example, another known superconductor includes a rare-earth metal (yttrium), a first-group metal (copper) and a second-group metal (barium), its composition being (Y.sub.0.9 Ba.sub.0.1) CuO.sub.y at T.sub.initial =95K, T.sub.final =75K and .DELTA.T=20K. To obtain the aforesaid superconductor, source powders of Y.sub.2 O.sub.3 :BaCO.sub.3 and CuO taken in a proper weight proportion are heat-treated at 1000.degree. C. at the initial stage and then at 1100.degree. C. for several hours (cf. Hidenori Takagi, Shin-Ichi Uchida, Kohji Kishio, Koichi Kitazawa, Kazuo Fueki and Shoji Tanaka: "High-Tc Superconductivity and Diamagnetism of Y--Ba--Cu Oxides", Japanese Journal of Appl. Phys., Vol. 26, No. 4, April, 1987, pp L-320-L-321).
The superconductor obtained in compliance with the above method is a multiphase material with nonuniform distribution of phases in the bulk of the specimen. It is characterized by inclusion of source constituents such as Y.sub.2 O.sub.3, CuO and Cu.sub.2 O, in addition to the main superconducting phase. The known superconductor manufacturing method involves a long and multistage production process, another disadvantage thereof being great power consumption.
Another known Y--Ba--Cu--O superconductor has T.sub.initial =90K and T.sub.final =77K. The method of manufacturing said superconductor comprises preparation of a mixture of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO taken in appropriate weight proportions, compaction of said mixture to form a blank, its heat treatment at 900.degree. C. for 6 hours in a rarefied oxygen medium (at P=2.times.10.sup.-5 bar), grinding of the blank, its repeated moulding to obtain a pellet and subsequent heat treatment at 925.degree. C. for 24 hours.
The obtained superconductor is a low-quality material characterized by a heterogeneous phase composition with inclusion of carbon up to 0.1% by mass. Moreover, the afore-mentioned superconductor manufacturing method involves a long (up to 10 h) and multistage production process consuming much power (cf. M. K. Wu, J. R. Ashburn and C. J. Torng: "Superconductivity at 93K in a New Mixed-Phase Y--Ba--Cu--O Compound System at Ambient Pressure", Phys. Rev. Letters, Vol. 58, No. 9, March, 1987, p 908).
Another known superconductor includes barium, copper and such a rare-earth metal as holmium (Ho). Its manufacturing method comprises preparation of a mixture of holmium oxide (Ho.sub.2 O.sub.3), copper oxide and BaCO.sub.3 in two constituent ratios, the atomic ratio of Ho:Ba:Cu being EQU 0.246:0.336:1 (1) EQU and EQU 0.316:0.335:1 (2)
With each ratio (1) and (2), the constituents are mixed up and heat-treated at 850.degree. C. for 2 h in an air envirionment whereupon the mixtures are formed into specimens measuring 8.times.2.times.1.5 mm, said specimens being subsequently sintered at 800.degree. C. for 1 h (cf. Shinobu, Hikami, Seiichi Kagoshima, Susumu Komiyama, Takashi Hirai, Hidetoshi Minami and Taizo Masumi: "High Tc Magnetic Superconductor: Ho--Ba--Cu Oxide", Japanese J. of Appl. Phys., Vol. 26, No. 4, April, 1987, pp L-347-L-348).
The superconductors obtained by the known method are characterized by EQU T.sub.initial =93K, T.sub.final =75K (I) EQU T.sub.initial =74K, T.sub.final =47K (II)
Great spread in .DELTA.T (superconducting transition range) is attributable to a heterogeneous phase composition and the low superconducting phase content of the superconductor. Moreover, the aforesaid manufacturing method entails a long and energy-intensive production process.
Other known superconductors contain Yb, Lu or La as a rare-earch metal, Ba or strontium as a second-group metal and copper as a first-group metal, the composition thereof being (Ln.sub.1-x Mx).sub.2 CuO.sub.4-.delta. where M is Ba or Sr.
The method of manufacturing such a superconductor comprises preparation of a mixture of Yb.sub.2 O.sub.3, Lu.sub.2 O.sub.3, Y.sub.2 O.sub.3 or La.sub.2 O.sub.3, BaCO.sub.3 of SrCO.sub.3 and copper oxide. Seven different compounds are prepared. The obtained mixtures are heat-treated at 900.degree. C. until green powder is produced. Thereafter the mixtures are formed into pellets which are subsequently heat-treated at 900.degree. C. for several hours until green colour changes into black (cf. Shoichi Hosoya, Shin-ichi Shamoto, Masashige Onodo and Masatoshi Sato: "High Tc Superconductivity in New Oxide Systems", Japanese Journal of Appl. Phys., Vol. 26, No. 4, April, 1987, pp L-325-L-326).
The obtained superconductors have T.sub.initial from 65 to 95K and T.sub.final from 26 to 60K, depending on the source composition of the mixture. Such superconductors have been generally unsatisfactory due to low quality, a wide superconducting transition range and a heterogeneous phase composition.
The foregoing superconductor manufacturing method entails a long, energy-intensive and inefficient production process.
Also known in the art are superconductors containing a rare-earth metal (scandium, yttrium or lanthanide), a first-group metal (copper) and a second-group metal (Ba or strontium) of the periodic system, the composition thereof being LnM.sub.2.sup.II Cu.sub.3 O.sub.y where M.sup.II is a second-group metal of the periodic system (Ba or strontium).
The method of manufacturing the aforesaid compound comprises preparation of a mixture of a rare-earth metal oxide, a constituent composed of a second-group metal (BaCO.sub.3 or SrCO.sub.3) and copper oxide taken in a ratio ensuring production of the end superconductor wherein the atomic ratio is as follows: rare-earth metal:second-group metal:first-group metal:oxygen=1:2:3:y. After mixing and grinding, the source constituents of the mixture are heat-treated at 950.degree. C. in an oxygen-containing medium for 12 h. The obtained black powder is mixed up and ground again. Next, said powder is heat-treated at 950.degree. C. under similar conditions. It is then formed into blanks at P=150 kg/cm.sup.3, said blanks being subsequently heat-treated at 700.degree. to 900.degree. C. in an oxygen medium for several hours (cf. E. M. Engler, V. Y. Lee, A. I. Nazzal, R. B. Beyers, G. Lim, P. M. Grant, S. S. Parkin, M. L. Ramirez, J. E. Vazquez and R. J. Savoy: "Superconductivity above Liquid Nitrogen Temperature: Preparation and Properties of a Family of Perovskite-Based Superconductors", J. Am. Chem. Soc., 1987, 109, 2848-2849). Disadvantage of the superconductors produced according to the above method are low quality, presence of contaminants such as source constituents and carbon, a wide superconducting transition range, great spread in parameters, and a small yield of a superconducting phase.
Moreover, the foregoing method involves a long, complicated, multistage and power-intensive production process.
This, the analysis of the prior art shows that, at the present time, there are no adequate methods of producing oxide superconductors containing a rare-earth metal, a first-group metal and a second-group metal of the periodic system, which would have stable characteristics, high quality, purity as regards inclusion of source constituents, a high yield of a superconducting phase (at least 70%, as indicated by permeability measurements) and its uniform distribution in the bulk of the specimen, a narrow (up to 1K) superconductivity zone, desired porosity (from 20 to 50%) and a controlled particle size.
The known methods of obtaining superconductors entail a long (up to 30 h) and energy-intensive production process and have low efficiency. Their automation and control present considerable difficulties.
With all the prior-art methods, source constituents of a mixture, that is, rare-earth metal oxide, second-group metal oxides or their carbonates and copper oxide represent compounds characterized by a maximum level of oxidation of the constituent metal. Stated differently, said mixture and its constituents are noncombustible components.